Your ds is the one who has to be there every day. I don't know what exh's other reasons are, but I would prioritise the feelings of your ds TBH.

I would fully brief ex and tell him that ds will be talking to him about it, explain to ex that ds is worried about exs reactions and needs his support. Any parent worth their salt would be horrified if their child was scared of their reaction (although I expect it happens often). I would make sure you have a really good talk with DH. Good luck.

i posted beginning of the year about ds being bullied at school (just started at secondary), I really wanted him to move to other school to be with all hi s friends but ex h really wanted him to stay at this one because dd is there and is happy, and other reasons.
So, we moved him into another class and things have got better, BUT, he is still not happy but I think he is scared to tell us because of ex reaction.
He says he is alright but I can see he is still missing all his other friends like crazy and my heart literally hurts with such a massive lurch every time I think about how I've let him down by not making the move happen sooner when he wanted to .
Please help me to help him and tell my ex that this is the way to go.